This application claims the priority benefit of Taiwan application serial no. 90132481, filed Dec. 27, 2001.
1. Field of Invention
The present invention relates to technology of light emitting device. Particularly, the present invention relates to a structure of light emitting diode using nitride-base III-N group compound. More particularly, the present invention relates to a structure having a reversed tunneling layer (RTL).
2. Description of Related Art
In recent years, gallium nitride-based III-N group compound semiconductor device, such as GaN, GaAlN, and GaInN, has been greatly taken as a light emitting device. FIG. 1 is a cross-sectional view, schematically illustrating structure of a conventional light emitting diode made of III-N group compound.
The light emitting diode is formed on a substrate 10, such as an Al2O3 substrate. A nucleation layer 12 and an N-type conductive buffer layer 14 are sequentially formed over the substrate 10. The buffer layer 14 includes GaN doped with N-type dopant, so as to ease the crystal growth for the subsequent crystal growing process. There is a light-emitting active layer 18 over the buffer layer 14. Usually, the active layer 18 is confined by a confinement layer, that is also, cladding layers 16, 20. The confinement layers 16, 20 are doped with opposite conductive type. For example, if the lower confinement layer 16 is the GaN layer doped with N-type dopants, the upper confinement layer 20 is the GaN layer doped with P-type dopants. Then, a contact layer 22 is formed on the upper confinement layer 20. The contact layer 22 is a P-type GaN layer. A transparent electrode layer 24 is formed on the contact layer 22. In addition, an electrode layer 26, serving as a cathode of the diode, is formed over the buffer layer 14 at the separated region from the confinement layers 16, 20 and the active layer 18.
In the above structure, the contact layer 22 is GaN layer doped with P-type dopants, where the dopants includes group II elements, such as Mg, Zn, Cd, Be. The dopants in GaN has a quite large activation energy. As a result, it is difficult to have high hole concentration in the contact layer 22. In addition, the P-type dopantsproduce carrier of holes, which have larger effective mass than that of electrons, causing a poor penetrability for the carrier. This also causes a poor ohm contact between the P-type contact layer 22 and the anode layer 24.
FIG. 2 is a cross-sectional view, schematically illustrating a light emitting region for the light emitting diode in FIG. 1. In FIG. 2, when the electrodes 24, 26 are applied with a forward bias, the diode is conducted. At this situation, current can flow from the electrode 24 to the active layer 18. In the conventional manner, the P-type contact layer 22 of GaN cannot have high carrier concentration and has large contact resistance between layer 22 and electrode 24. This results in a poor quality of current spreading. The p-type electrode layer 24 also only covers a portion of the contact layer 22. As shown in FIG. 2, the area having current flow is about the width L of the electrode layer 24. This limits the light emitting area for the diode. The function of the active layer cannot be fully performed. The light emitting efficiency of the diode is then greatly reduced.
In summary, the conventional light emitting diode is restricted by the physical properties of the contact layer. The P-type contact layer is difficult to grown with high hole concentration. This also causes the high fabrication cost and also causes low yield. Further still, the conventional structure cannot provide a diode with high light emitting efficiency. A large portion of the active layer 18 of the diode is not well utilized.
The invention provides a structure for a light emitting diode. The invention forms a reversed tunneling layer (RTL) with a high doping concentration on the contact later. The RTL associating with the transparent electrode can improve the light emitting efficiency of the product and reduce the operational voltage.
The invention provides a structure for a light emitting diode, which uses a reversed tunneling layer with high doping concentration on the contact layer, so as to improve the ohmic contact between the transparent electrode and the RTL.
As embodied and broadly described herein, a structure for a light emitting diode of the invention is as follow:
A structure for a light emitting diode includes a substrate. A nucleation layer and a buffer layer with a first conductive type are sequentially formed on the substrate. A confinement layer (lower confinement layer) with the first conductive type is disposed on the buffer layer. The confinement layer with the first conductive type and the conductive buffer layer have the same conductive type, such as P-type or N-type dopants. The active layer is located on the confinement layer to serve as the light emitting layer of the light emitting diode. A confinement layer (second confinement layer) with a second conductive type is disposed on the active layer. The confinement layer with the second conductive type and the confinement layer with the first conductive type have different conductive type. The contact layer with the second conductive type is located on the second confinement layer. The conductive types for the doped contact layer and the second confinement layer are the same. The reversed tunneling layer is located above the contact layer. The conductive types for the reversed tunneling layer and the contact layer are different. The transparent electrode is located above the reversed tunneling layer, and then an anode layer is formed. Another electrode, serving as a cathode layer, has a contact with the conductive buffer layer, and is separated from the upper/lower confinement layer, the active layer, the contact layer and the transparent electrode.
The foregoing dopants in the contact layer are P-type dopants as the conductive type. The N-type dopants for the reversed tunneling layer includes suitable N-type ions, such as Si+, P+, As+, Se+, Te+, S+, O+, and so on.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.